1. Field of the Invention
The present invention relates to image processing and recognition systems. More specifically, the present invention relates to license plate locators and readers.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
For many applications, there is a need for a system for reading a license plate of a motorized vehicle. One such application involves the automated monitoring of traffic flow to identify vehicles traveling at excessive speeds. Another application involves the automatic identification of vehicles passing through toll booths without payment of the toll.
Many systems are known in the art for reading license plates. See, for example, "Development and Application of Vehicle-license Number Recognition System Using Real-time Image Processing", by K. Kanayama et al, published in Systems and Computers in Japan, vol 22, no. 1, p. 49-58, 1991.; "Automated License Plate Reading", by L. Howington, published in Advanced Imaging, September 1989; "Character Recognition in Scene Images", by A. Shio, Proceedings of AUTOFACT '89, Detroit Mich., 1989; "A Method of License Plate Extraction Using a Fast Pyramid Hierarchical Hough Transformation", by T. Agui, H. Jin Choi, and M. Nakajima, Systems and Computers in Japan, vol 19, no. 8, pp. 69-76, 1988; and "Gray Scale Image Processing Technology Applied to Vehicle License Number Recognition System", by M. Takatoo et al., published in Proceedings of the IEEE International Workshop on Industrial Applications of Machine Vision and Machine Intelligence, pp. 76-79, Tokyo, Japan, 1987.
See also the following patents, "License Plate Recognition System for Moving Car", pat. no. GB 2246894; "License Plate Character Recognition Apparatus", pat. no. GB 2217498 and U.S. Pat. No.4,878,248; "Character Recognition System for e.g. Vehicle Number Plate Monitoring", U.S. Pat. No. 4,567,609; "Apparatus and Method for Reading a License Plate", U.S. Pat. No. 5,081,685; Finding and Identifying Apparatus for Image of Car License Plate", WO 8707057, U.S. Pat. No. 4,817,166, EP 310611, JP 1502371.
In addition, Computer Recognition Systems of Massachusetts and Perceptics of Knoxville, Tenn. manufacture and sell automated license plate recognition systems which represent the current state of the art.
As is evident from these references, conventional license plate reading systems separate the pixels of the image of the plate that makeup characters from the pixels of the image that create background imagery. Each of the segmented pieces is then analyzed to determine what type of character (e.g. numeral or letter) is present based on the features in the image.
Unfortunately, the traditional correlation approaches utilized in conventional license plate reading systems have difficulty distinguishing between certain characters.
In addition, system design is complicated by the fact that the camera must be placed to the side of the road. This creates perspective distortion which forces a compromise. That is, to minimize perspective distortion, the camera must be placed at a distance from the vehicle of approximately fifty feet. Unfortunately, at lower traffic speeds and at this distance, the image of one vehicle is often blocked by a following vehicle.
Thus, there is a need in the art for a system and technique for reading license plates which has a high probability of correct read which accommodates perspective distortion.